The ruh is the spirit which is the loci of life. It is composed of the will to reconcile with God. It inclines towards righteousness, piety, etc. The qalb is the component, often translated to heart, that balances between the two keeping in check feelings of selfishness and selflessness so one maintains a calibrated scale of both in character.
Angels are composed simply of ruh because they don't need to eat, sleep, procreate, etc. and thus have no requirement for a control center of such desire in place. They are immortal spirits and thus they have no nafs and since they have no nafs they cannot sin because sinning is caused by the unchained passions of the nafs, that is, when the nafs desires for food and the person in possession of the nafs satisfies this desire by eating that which is not lawful, it is a sin, or when the nafs desires to procreate and the person in possession of the nafs satisfies this desire by immoral sexual intercourse, it is a sin. Angels do not and they cannot sin under any circumstances.
Since angels have no requirement to eat or procreate, etc. they have no nafs, hence their inability to sin. They only consist of ruh, or the spirit, and thus they are pure righteousness and piety incarnate. The jinn, on the other hand, are capable of sinning because, like humans, they too are mortal and require instincts for survival thus they come off the factory settings with both the nafs and ruh in place. Iblis, before his fall, was a pious jinn, Azazel, but due to his disobedience against God was stripped of his grace and morphed into the Accursed Satan.
— Fahim Ferdous Promi